Mg-si-zn extrusion alloy



llniteti; rates Patent 3,ll67,23t5 lrlg'Si-Zn EXTRUMUN ALLOY George 5. Foerster, Midland, Mich, assignor to The Dow llhemical Company, Midland, Mich, a corporation of Delaware No Drawing. Filed Apr. 27, 1960, Ser. No. 24,889 3 Claims. (@l. 75-168) The invention relates to a magnesium-base alloy and more particularly relates to an improved magnesiumbase extrusion alloy containing silicon and zinc. These alloys can be extruded at speeds as high as 100 feet per minute.

Heretofore, the magnesium-aluminum-zinc alloys have been employed as the principal extrusion alloys because they are readily extruded at speeds up to 75 feet per minute, they exhibit moderately high physical properties in the extruded form and they are prepared using low cost alloying elements.

However, the conventional alloy AZ31B containing nominally 3 percent of aluminum, 1 percent of zinc, at least 0.2 percent of manganese and the balance magnesium, is not weldable without stress relief. Upon lowering the aluminum content of a magnesium-aluminumzinc alloy the physical strength properties of the extruded alloy are found to be inferior to those of AZ31B. Such is the case with the magnesium-base alloy A211 which contains nominally 1 percent of aluminum, 1 percent of zinc, the balance magnesium.

In considering other possible magnesium-base extrusion alloys it is well recognized that the addition of increasingly larger proportions of zinc to magnesium, or a magnesium-base alloy, tends to increase the strengths of the resulting alloy. Typical trends are illustrated by the strength properties listed in Table I for extrudes formed of magnesium-zinc binary alloys. The data in the table show the expected increase in properties with increasingly larger proportions of Zinc in the alloy and lower properties at higher extrusion speeds.

Other alloys such as the magnesium-aluminum-zinc alloys are known to behave similarly.

it has now been discovered that the strength properties of a magnesium-base alloy containing silicon and zinc are unexpectedly enhanced by providing a reduced amount of zinc, i.e. below about 1.5 percent by weight, in combination with a silicon content in the range of 0.2 to 2 percent by Weight.

The alloy of the invention is readily prepared accord ing to methods well understood in the magnesium metal art, Magnesium is melted under a conventional flux cover and the requisite amount of silicon and zinc added to the melt while the melt is at a temperature of about 1600" F.

Ferrosilicon is usually the source of silicon, although other forms of silicon such as silicon metal may be used,

if desired. In the event ferrosilicon is employed, iron, which is sparingly soluble in molten magnesium, normally precipitates into the bottom sludge layer of the melt. While the zinc may be added to the melt at any time, if ferrosilicon is the source of silicon, the requisite amount of the zinc is added to the melt as metallic Zinc after the iron settles out.

After the addition of the alloying components, the melt is brought to casting temperatures, usually in the range of 1250 to 1450" F., and cast into billets suitable for extrusion stock.

The role of the alloying elements is not entirely understood, but it is believed that the silicon addition precipitates in the casting as the finely divided intermetallic compound, magnesium silicide. Magnesium silicide has a very low solid solubility in magnesium and helps to maintain the alloy in the fine-grained condition as well as to increase the strength of the alloy. The silicon content of the alloy of the invention should be as high as feasible, preferably from 0.5 to 1.5 percent.

The presence of zinc greatly improves the corrosion resistance of the alloy and therefore at least 0.2 percent of zinc should be employed. On the other hand at higher zinc levels, such as above about 1.5 percent of Zinc, the tensile yield strength, compression yield strength, and ultimate tensile strength of the alloy are decreased. To impart ductility and to improve the surface of the ex- TABLE 1 Composition, Extru Strengths in 1,000 p.s.i. weight percent sion Form of Cross-soctional Percent speor l, ertrurle dimensions, E

lt./n1in. inches Zn Zr TYS CYS TS 0. 9 3 /s diameter 16 21 12 65 1.9 3 diameter 18 23 13 3 1. 3 0. b 12 as diameter 18 5 29 45 2v 4 0. 6 12 5s diameter 18 40 33 5O 1. 3 0. 6 12 Ms K 5 3% 29 41 2. 4 0.6 12 Mr; X 6 40 38 47 l. 3 0.6 Vs x 6 34 19 2.4 0.6 20 Ms K 8 38 29 2!) 1.3 0.6 40 Me X?/ 11 27 15 37 2.4 0.6 40 As it it 12 31 20 43 1 Balance magnesium.

yield strength.

TYS=Tensile yield stren th. CYS=Co1r1pression In the German Patent 519,377 there is described a trude without adversely alfecting weldability the zinc conmagnesiurn-base alloy containing from 1.8 to 5 percent by weight of zinc and from 1 to 6 percent by weight of silicon. The alloy of the German patent is said to be useful when cast as pistons for internal combustion engines. An example of this alloy such as one containing 4 percent by weight of zinc and 1.5 percent by weight of silicon, the balance magnesium is not, however, suitable as an extrusion alloy because it tends to crack on being welded, it cannot be extruded at speeds above about 30 feet per minute, and an extrude of the alloy does not exhibit the level of strength properties desirable in an extruded product.

tent of the alloy should preferably be in the range of 0.5 to 1.3 percent and more desirably about 1 percent.

it has also been found that various additional alloying elements may be added to the magnesium-silicon-zinc alloy of the invention to advantageously modify the properties of this alloy for special purposes.

An especially desirable additional alloying element is manganese. The addition of small amounts of manganese permits the use of a lower silicon content without loss of mechanical strength. This is often advantageous as silicon tends to not alloy efficiently with molten magnesium, and lower silicon contents are more emily obtained. Up to 1.5 percent by weight of manganese can be incorporated into the alloy to increase the mechanical strength and corrosion resistance thereof. A preferred composition is one containing by Weight, about 1 percent of silicon, about 1 percent of zinc, about 0.4 percent of manganese, the balance magnesium.

Beryllium in an amount up to 0.05 percent by Weight may be added to increase matrix strength and to improve resistance to oxidative attack by the atmosphere. Up to 0.5 percent of misch metal may be added to increase matrix strength and cold workability. Up to 0.5 percent of calcium may be added to increase matrix strength where some brittleness may be tolerated. Up to 0.1 percent of zirconium may be added to obtain beneficial grain refining action. And, up to 3 percent of aluminum may be added to improve the surface appearance of the resultant extrude.

Magnesium-silicon-Zinc alloys prepared in accordance with the invention can be extruded at speeds as high as 100 feet per minute and at reduction ratios as great as 150 to l. Extrusion speed does not ordinarily adversely aifect the physical properties of the extruded material nor is the extrusion subject to hot shorts.

EXAMPLES In accordance with the present invention, a series of magnesium-silicon-zinc alloys were prepared by alloying together the requisite proportions of magnesium, ferrosilicon, and zinc metal. The resulting alloys were separated from the precipitated iron and each cast as 3 inch diameter billets. The cooled solidified billets were scalped, heated to about 700 F., placed in a preheated ram-extrusion press, and extruded at a temperature of about 600 F. and at speeds in the range of 5 to 100 feet per minute. The physical properties of the resulting extrusions were measured. In Table II are listed the physical properties for 2 alloys of the invention which were each extruded at different speeds. The data show that high extrusion speed is not deleterious but even beneficial to the physical properties of the extruded product.

TABLE II Effect of Extrusion Speed 1 Composition in Weight percent, balance magnesium.

The physical properties of other alloys of the invention which were extruded at 100 feet per minute are listed in Table III. Some of the alloys listed in Table III were also tested to determine their resistance to corrosion. In these instances corrosion tests of one week duration were carried out by the alternate immersion method according to which test specimens are repeatedly immersed for 30 seconds in a 3 percent aqueous sodium chloride solution maintained at 95 F., then withdrawn from the chloride solution for 90 seconds on a two minute cycle according to ASTM standard IB (B 192-44T).

By way of comparison extrudes formed, respectively, of a magnesium-silicon binary alloy, of a magnesium-zinc binary alloy, and of the conventional alloys AZ3lB and A211 were tested to determine their physical properties and corrosion resistance. The results of these additional tests are also listed in Table III for comparison.

In accordance with the invention, a further series of magnesium-silicon-zinc alloys containing a fourth alloying element selected from the group consisting of manganese, misch metal, calcium, zirconium, beryllium, and

TABLE III Composition} percent Physical properties Extrusion Run No. speed, 1,000 p.s.i. Corrosion S1 Zn Al Mn feet/min. Percent rate,

E M/C/D TYS OYS TS Comparison 1. 4 100 5 22 17 37 Ca.

D0 3 0. 4 50 15 29 14 38 Ca. 0 4

D0 0.6 1. 3 .3 100 11 23 11 34 )16 Do 1 60 8 14 7 28 Ca. 1

1. 4 100 9 22 16 35 1. 4 10 21 15 1. 4 Very poor, severely erae ed as extruded 1 Composition in weight percent, balance magnesium. No'rE.Percent E=Perccnt elongation in 2 inches. OYS=Compression yield strength in thousands of p.s.i.

2 Typical range. TYS=Tensile yield strength in thousands o1 p.s.i. TS=Ultimate tensile strength in thousands of ps i.

M/O/D=Milligrams corrosion loss per sq. centimeter of surface area per day.

aluminum, were prepared by alloying together the requi- I claim:

site proportions of magnesium, ferrosilicon, zinc and the l. A magnesium-base alloy consisting essentially of fourth metal. The resulting alloys were separated from rorn 0.2 to 2 percent by weight of silicon, from 0.2 to 1.5 the precipitated iron and each cast as 3 inch diameter percent by weight of zinc, the balance being essentially billets.

The cooled solidified billets were scalped, and 5 magnesium. heated to about 700 F. and placed in a pre-heated ram 2. A magnesium-base alloy consistlng essentially of extrusion press and extruded at a temperature of about from 0.5 to 1.5 percent by weight of silicon, from 9.5 to 600 F. and at speeds in the range of 5 to 100 feet per 1.3 percent by weight of zinc, the balance being essenminute. The physical properties of the resulting extrutiall magnesium. sions were determined and are listed in Table TV. 19 3. A magnesium-base alloy consisting essentially of TABLE IV Composition, percent Physical properties Run Listen 4 No. speed, 1,600 psi.

Si Zn Be Mn Z1 Al MM feet/min. Per ent TYS CYS TS 17.-. 1.5 100 2s 1s 37 18... 1.2 100 30 1s 3 0.-. 0. 1 1. 0 o, 4 100 12 22 11 a6 21... 0. 9 1. 0 0. 4 .1 100 12 26 1s 39 22--. 1. 4 1. u 0. 2 100 8 25 1s 3? 1. 5 1. 0 o. 0 100 9 24 17 24-.- 1. 4 1. 0 Tr, .01 100 a 1s 37 25-.. 1. 7 0. s 0. 9 100 6 21 32 1 Composition in wcliht percent, balance magnesium. NOTE.-Percent E=Pcrcent elongation in 2 inches.

0 Y8 00 more; on yield st .ngth in thousandsotp MM=Misch metal, a ml. at rare earth metals;

TYS=Tonslle yield strength in thousands of psi. TS: Ultimate tensile strength in thousands of psi. mixture used contained about percent of cerium.

Among the advantages of the alloy of the invention are from 1 to 1.5 percent by weight of silicon, about 1 perthe improved corrosion resistance and ductility that it cent by Weight of Zinc, the balance being essentially exhibits compared to that of n1ag11esini.-silicon binary 35 magnesium. alloys.

This application is a continuation-in-part of a prior filed application, Serial No. 819,814, filed lune 12, 1959, now abandoned. 45,

References ilitcd in the file of this patent FOREIGN PATENTS 37 Denmark Oct. 4, 1929 Italy Mar. 3, 1945 

1. A MAGNESIUM-BASE ALLOY CONSISTING ESSENTIALLY OF FROM 0.2 TO 2 PERCENT BY WEIGHT OF SILICON, FROM 0.2 TO 1.5 PERCENT BY WEIGHT OF ZINC, THE BALANCE BEING ESSENTIALLY MAGNESIUM. 